The objective of this proposal is to determine whether the unique periodic clinical course of equine infectious anemia is due to "antigenic drift" of the persistently infecting EIAV and to characterize the biochemical nature of any antigenic alterations detected in the virion. Towards this goal, a variety of biochemical techniques developed and successfully employed with prototype retroviruses will be used to characterize the polypeptides of laboratory EIAV strains according to molecular weight, isoelectric point and glycosylation or phosphorylation, their relatedness as determined by peptide mapping, and their stoichiometry and location in the virion. Each structural polypeptide will be purified in quantities sufficient for the production of heterologous monospecific antiserum. These reagents will then be used to develop specific radioimmunoassays for EIAV proteins and to study the expression of virion polypeptides in host tissues by immunofluorescence and radioimmunoprecipitation procedures. Simultaneous with the above studies, wild-type EIAV and serum will be isolated from sequential natural and drug-induced febrile episodes in horses and the antigenicity of each EIAV isolate compared in cross-neutralization studies. To determine the nature and site of any detectable antigenic alterations, the polypeptides of the EIAV isolates will be characterized and compared by the immunologic and biochemical techniques developed with laboratory strains of EIAV.